Impression and compression molding, which have long been used in the manufacture of analog recordings, have also been used in the manufacture of video and compact discs. In these methods, pre-heated molten plastic is charged to a mold or "stamper" having a relief pattern corresponding to the micron-sized information-carrying pits required in the finished disc. Pressure causes the fluid polymer to flow into the pattern and conform to the stamper relief. Cooling produces a solid disc with the desired signal pattern, thickness and other dimensions required. The embossed discs are then finished by the application of reflecting and protective layers, labeling, etc., by known standard techniques.
U.S. Pat. Nos. 4,363,844 and 4,519,065 describe a closely related process for making video discs and similar devices. The starting composition is not molten plastic, but can be an element comprising a very thin embossable radiation reflective layer overlying an embossable, heat softenable layer which can be simply thermoplastic or can also be radiation curable. Optionally, the heat-softenable composition can be coated on a substrate. The heat-softenable composition is chosen so that its maximum loss modulus is between 30.degree. and 180.degree. C., which allows embossing at 50.degree.-200.degree. C. at pressures of 5-100 kg/cm.sup.2. These conditions are similar to those used in compression or injection molding techniques for making video and compact discs. Impressing the stamper information into the heat softenable layer can be done with a platen or roll embosser. Radiation curing helps retain the desired relief shape by crosslinking. Discs are provided with the reflective layer before they are impressed with the information carrying relief pattern.
A significant disadvantage with both of the high pressure, high temperature relief-forming methods described above is the potential for image distortion and internal stresses in the discs produced.
As described in the Journal of Radiation Curing, a photopolymerization method was developed to help overcome these limitations (A. J. M. van den Broek et al., Journal of Radiation Curing, 11, 2-9, 1984). In this method which is frequently referred to as the "2P" process, a measured amount of liquid, low viscosity photopolymerizable acrylate monomer or mixture of acrylate monomers is applied at room temperature to the information-carrying master, spread out uniformly by applying to the liquid layer a flexible film of transparent plastic which is suitable as a permanent disc substrate. After the liquid fully covers the mold in the proper thickness, the liquid composition is photohardened and adhered to the substrate by exposure to ultraviolet radiation through the substrate.
Because the method uses a low viscosity liquid composition, high temperature and pressure are not needed to make the material accurately conform to the stamper. Further, the stampers used need not be as mechanically strong as in compression molding methods which allows the use of lower cost plastic stampers.
In U.S. Pat. No. 4,296,158, mixtures of polyacrylate monomers and heterocyclic group-containing oligomers with terminal unsaturation are spread over a stamper and simultaneously covered with a polymeric film by means of a pressure roll. The film becomes the disc substrate.
In U.S. Pat. No. 4,354,988 a process is described that produces optical discs by combining the steps of spreading photocurable resin on a stamper, covering with substrate film, curing, punching a center hole and trimming, all in a continuous sequence.
U.S. Pat. No. 4,482,511 discloses a similar approach using radiation-sensitive liquid compositions comprised of monomers, oligomers or mixtures thereof. Pressures of less than 1 kg/cm.sup.2 are needed and high fidelity glass master originals can be used. Means for applying a thin fluoropolymer release layer to the stamper surface is also disclosed.
In U.S. Pat. No. 4,510,593, polymerizable mixtures of mono- and polyunsaturated monomers also containing soluble film-forming polymer binders are described which are applied to a substrate then contacted with a flexible stamper by means of a pressure roll. The presence of the binder is stated to improve the properties of the information-carrying layer. In U.S. Pat. No. 4,430,363, a screen printing method for applying similar radiation sensitive mixtures to the substrate is described. In both of these patents, curing is achieved by exposure through an actinic radiation-transparent stamper.
In U.S. Pat. No. 4,582,885, a fluid, polymerizable oligomer composition containing "hard" and "soft" segments that can be varied to control end-use physical properties is used to prepare a variety of information-carrying articles, including optical discs.
While the liquid compositions described in the above patents and technical article may be fluid enough to conform to the information-carrying stamper relief pattern at room temperature under application pressures much lower than required for compression molding, they nevertheless have certain inherently practical limitations: (1) fluid compositions require a precision mold to contain the liquid; and (2) careful control of the amount of liquid charged during each cycle is required to maintain uniformity of thickness and other critical disc dimensions.
On the other hand, U.S. Pat. No. 4,790,893 to Watkins discloses a process for making information carriers such as CD's in which a thermoplastic material layer is extruded at a temperature above its softening point onto a patterned metal master. Pressure is applied to the layer which is cooled and the cooled layer is separated from the metal master. A thin metal film is applied to the patterned layer and the patterned layer is laminated to a substrate.
In prior art elements where the photocurable composition is applied as a layer on a substrate, either before being impressed by the stamper, or if applied to the stamper then contacted by the substrate, careful attention must be paid to the radiation curing step to assure adequate substrate adhesion. Overcuring can lead to excess information layer shrinkage, variable information distorting internal stress or unproductive long exposure times. The alternative would be severe restrictions on the selection of acceptable compatible combinations of substrates and curable compositions.